V-shaped magnetic separator assembly



NOV. 1966 c. D. SLOAN V-SHAPED MAGNETIC SEPARATOR ASSEMBLY ATTOR EY 2Sheets-Sheet 1 INVENTOR Carroll D. Sloan BY Filed July 18, 1963 Nov. 8,1966 c. D. SLOAN V-SHAPED MAGNETIC SEFARATOR ASSEMBLY 2 Sheets-Sheet 2Filed July 18, 1963 United States Patent 3,283,900 V-SHAPED MAGNETICSEPARATOR ASSEMBLY Carroll D. Sloan, Murrysville, Pa., assignor toWestinghouse Electric Corporation, Pittsburgh, Pa., 2 corporation ofPennsylvania Filed July 18, 1963, Ser. No. 296,041 Claims. (Cl. 209223)This invention relates to magnet assemblies particularly useful, by Wayof example, in magnetic separators for the resolution of magnetic oresinto a desired fraction and a gangue fraction. It also concernsapparatus that includes such magnet assemblies.

There are presently available commercially a number of dilferent typemagnetic separators. In those devices, which are of a drumconfiguration, permanent magnets and pole pieces are arranged relativeto an outer surface of the drum, designated the separation surface, toexert a magnetic influence along that surface whereby magnetic ores, forexample, are attracted and held there and can thus be separated fromremaining constituents, commonly designated as gangue. The magneticconveyor or surface generally is rotated relative to the magnetarrangement and the separated materials are removed from the gangue to aplace where they can be collected.

Analysis of the prior art magnet arrangements in those devicesdemonstrates the existence of significant drawbacks by Which about /2 to/3 of the magnetic fiux is not utilized. Thus, in many such devices,substantial spaces are located between the permanent magnets andmagnetic flux exists in those spaces. Since the spaces are within thedrum and between adjacent magnets, it is evident that this internal fluxconstitutes a leakage or a loss of flux compared to the total thatshould be available to effect magnetic separation. In still otherdesigns, the spaces between magnets have been avoided by placing thepole pieces therein; that eliminates internal flux between the magnets.However, in that arrangement, magnetic flux passes between the polepieces rearwardly of the magnets. Hence, while a portion of the fluxextends externally of the conveyor surface and is useful, another andsignificant portion appears rearwardly of the magnet and pole assemblyand thus is wholly removed from any area where it can exert a usefulinfluence for the intended purposes. That arrangement, therefore, simplyrelocates internal flux leakage and does not eliminate it. Similarproblems exist with regard to magnet chucks or assemblies used inconveyors, in sheet spreaders and other applications.

The present invention is an improvement on the device disclosed in mycopending application Serial No. 254,223, filed January 28, 1963, now"US. Patent No 3,209,912, issued October 5, 1965.

It is the primary object of the present invention to provide anarrangement of magnets and pole pieces particularly useful in magneticapparatus and in which optimum utilization of magnetic material isapproached in that the flux density in all sections of the magnetmaterial closely corresponds to that of the maximum energy product.

Another object is to provide magnetic separator, conveying and spreadingapparatus including an assembly of permanent magnets and pole pieces inaccordance with the foregoing object.

Other objects and advantages of the invention will be apparent from thefollowing detailed description and drawings, in which:

FIGURE 1 is a diagrammatic illustration of a magnetic drum separator;

FIG. 2 is a perspective view away, showing a permanent magnet assemblyfor use in a drum separator, the

3,283,900 Patented Nov. 8, 1966 assembly of magnets being arranged inaccordance with this invention;

FIG. 3 is a sectional view taken along line III-III of FIG. 4; and

FIG. 4 is a sectional view taken along line IV--IV of FIG. 3.

Basically, the objects are attained in the present invention by a magnetassembly including at least one pair of permanent magnets, or at leastone pair of stacks of permanent magnets, that are arranged,magnetically, in series.

Broadly, the magnetic assembly of this invention is a structure composedof soft magnetic pole pieces or members and pairs of permanent magneticmaterial members or bodies, shaped, arranged and assembled to provide afirst elongated pole face of one polarity, and a second elongated poleface of the opposite polarity spaced from said first pole face andsubstantially parallel there to. The pole faces, defining theextremities of the working face of the magnetic assembly, usually arearranged parallel to a plane or curved surface of large radii-us.

More specifically, the magnetic assembly of this invention ischaracterized by a V-shaped yoke of low reluctance, the arms of the Vextending rearwardly away from the working face of the assembly andhaving magnetic bodies fitted on the forward side thereof, in which thearms of the yoke provide a low reluctance path between the magnet bodiesand connect said magnet bodies in series. Pole pieces are arranged atthe end of the magnetic bodies opposite to the end in contact with theyoke. The pole pieces and yoke are held in fixed spaced relation to eaohother and to the bodies of the magnetic material which are securedbetween them. The bodies of permanent magnet matenial are positioned soas to confer one polarity upon one magnetic pole piece and the oppositepolarity upon the other pole piece. Thus, the yoke supports the rearsurfaces of the magnet bodies and the pole pieces abut the forwardsurfaces of the magnet bodies. The magnet bodies are magnetized in adirection that is perpendicular to the planes of the aforementionedsurfaces.

Accordingly, the yoke provides a pair of inclined convergent surfacesforming an apex in the direction of the working face. The magnet bodiesare fixed to the forward side of these inclined surfaces and extends tothe working face of the assembly. Consequently, the yoke mayalternatively be formed of a solid bar of triangular, square, sector orrectangular cross section.

It will be understood that the yoke arms may be flat or curved surfaceswith correspondingly shaped magnet bodies fitting closely thereagainst,and the pole pieces also fitting into the space at the upper end of themagnet bodies.

In utilizing the basic assembly described, ordinarily a plurality ofassemblies will be used depending upon the number of poles desired. Forexample, a five-pole arrangement may be obtained by employing four ofthe described yoke-magnet bodies and pole piece assemblies. In arelatively simple fashion, a magnet assembly is achieved in which theflux density in all sections of the magnet body closely correspond tothat of the maximum energy product. Further the efficiency achievedapproaches percent, based on the better utilization of the magnetic fluxrelative to the working face, for the structural and magneticarrangement of the present invention eliminates substantial open spacesbetween permanent magnets and concentrates and conducts the magneticflux into the area where it can be utilized ahnost entirely to effect adesired object, for example, a magnetic separation.

In a preferred embodiment, the magnetic assembly of this invention takesa form which may be designated the'Vtype. In this type, the yoke is-Vshaped in cross section with the arms of the V set at about 90 withrespect to each other and the pole pieces are triangular in crosssection with one surface thereof parallel to one surface of said yoke. Apair of magnets or a pair of stacks of slab-shaped magnets are securedbetween the yoke and the pole pieces. The magnets associated with eacharm'of the yoke are magnetically in series and'the yoke provides a lowreluctance coupling so that all the magnets of the assembly aremagnetically in series.

' The invention will be further described in detail with reference'tothe utilization of V type magnetic assemblies in drum type magneticseparators for resolving magnetic ores. However, it is to be understoodthat this is for purposes of illustration and is not to be construed aslimiting the invention.

Referring now to the drawings, the magnetic separator shown comprises asupporting frame on which is mounted the active separator unit indicatedgenerally by the numeral 12. As will be apparent in the detaileddescription hereinafter, the separator unit structually comprises agenerally cylindrical drum 13 of non-magnetic material (such asstainless steel) surrounding an assembly of magnets and pole pieceslocated along, but spaced from, a portion of the inside surface 14(FIGS. 3 and 4) of the cylindrical Wall 15 of the drum 13. The magnetarrangement is supported independently of the drum'13 so that the drumcan be rotated while the assembly of magnets remains stationary.

Accordingly, the drum 13 is supported on the frame '10 by a shaft 16. Amotor 17 operatively connected to the drum 13 through a sprocket andchain arrangement 18 attached to the end walls 19 of the drum 13 is alsosupported on frame 10. Shaft 16 suitably supports the drum 13 by meansof bearings 19a attached to the end walls 19.

, Internally of frame 10, there is structure by which I feed to beresolved is directed to the external surface of the cylindrical wall 15of the drum of the magnetic separator unit, and by which the fractionsinto which the feed is resolved can be separately removed. For thisgeneral purpose, a conduit 22 leads into an L-shaped chamber 24 at oneside of the frame 10. The materials in the feed that are non-magneticand thus not influenced by the magnet assembly pass downwardly along theL-shaped chamber 24 and are withdrawn through an outlet conduit 25 andare stored or otherwise disposed of as desired. The magnetic materialsin the feed are attracted and held to the external surface of thesidewall 15 of the drum until they are carried beyond a point where theycould fall into the path to the outlet conduit 25. Once they have beencarried beyond this point, they are permitted to drop into a bin oroutlet conduit 26 providedto receive them.

Within the drum 13 is the permanent magnet assembly, indicated by thenumeral 30 in FIG. 2. The arrangement of the permanent magnets and polepieces in this embodiment of the invention is shown in FIGS. 2 and 3.This unit includes a pair of supporting end plates 32 and 34 havingcross bar means 65 and 66, respectively, associated with each by whichthe magnet assemblies can be supported. If desired, adjustable means,such as setscrews (not shown) may be provided to lock the end plates 32and 34 in any desired fixed position on shaft 16. The permanent magnetsand pole pieces preferably are maintained as close as possible to theinside surface 14 (FIG. 3) of the drum side wall consistent with thefree rotation of the drum relative to the magnet assembly.

An arrangement of permanent magnets to operate, magnetically, in seriesin accordance with this invention is achieved by aligning at least twopermanent magnets, or two stacks thereof, generally to a V form. In theembodiment shown, each stack of magnets is composed of four slab magnets41, 42, 43 and 44 of increasing width from bottom to top. The actualwidth of each magnet preferably is determined to that available volumebetween the yoke and the pole pieces is substantially entirely filledwith magnetic material and voids, if any, are minimized. A pole piece 48of the shape to be received in the V defined by the smallest magnet slab41 and the working face of the magnet assembly is located therein.

The magnet assemblies as just described may be made and employed asindividual assemblies and conventional means may be used to secure theseveral elements together. However, in devices employing several magnetassemblies, it is convenient and desirable to construct the device sothat adjacent magnet assemblies share pole pieces and means for securingthe elements together.

In FIGS. 2 and 3, there is shown a five pole magnet strip assembly,which includes a plurality of magnet assemblies secured together andaffixed to the end plates 32 and 34 of the magnetic separator. Thedevice shown is provided with four magnet assemblies 7 ii, 71, 72, 73each having a yoke 50, 51, 52 and 53. The two middle magnet assemblies71, '72 share three pole pieces 47, 48 and 49 which are triangular incross section while the end magnet assemblies 70, 73 each have one polepiece 45 and 46, respectively, which is not shared. The magnetic stripis held together in part by a plurality of threaded means 80, locatedbetween adjacent magnet assemblies 71 and 72, which is threaded at oneend into a pole piece 47 and at the other end is provided with a collar81 which extends into abutting relation with the rearward edge ofadjacent arms of the yokes 51, 52. It will be noted that by tighteningthe nut 82, the magnet stacks of adjacent magnet assemblies can be drawnfirmly to their respectivelyokes. The support of the flank magnet stacksof the end magnet assemblies 70 and 73 requires that a hole 85 bedrilled in one arm of the yoke and the magnet stack so that thefastening means 80, 82 may effectively draw the pole piece 46 and themagnet stack to the yoke.

To secure the magnet assemblies to the end plates 32 and 34, thehorizontal cross bars 65 and 66, respectively, are welded or otherwisesecured thereto. The cross bar 65 is provided with downwardly extendingstruts 61 and 62, and the cross bar 66 is similarly provided with struts63 and 64. The struts are welded or otherwise secured to the cross bars.As will be seen in FIGS. 2 and 3, the ends of the cross bars 65 and 66closely fit into the angle provided by the rear surfaces of the endyokes 50 and 53. The cross bars are preferably welded to the yokes.Downwardly extending struts 61 and 63 closely fit into the 90 angleprovided by the rear surfaces of yoke 51. Similarly, the struts 62 and64 mate with the rear surfaces of yoke 52. The struts are preferablywelded to the yokes. It is clear that an extremely strong structure hasthus been provided.

In the embodiment shown in FIG. 2, there are four V arrangements ofmagnets that, in the aggregate and in view of their arrangement,contribute five poles. Additional magnet assemblies each contribute onemore pole, and accordingly, any odd or even number of poles desired canbe easily provided. Where a 360 arc of magnet assemblies is employed, aneven number of poles is obtained, for the final magnet assembly merelycloses the circle and contributes no separate pole. While the magneticassembly of the invention has been described primarily in terms of a Vconfiguration, in which the yoke arms are fixed at a 90 angle to eachother, other angles, smaller or greater, may be used.

To assure optimum utilization of permanent magnet material, it isnecessary that all sections of the material be operated at a fluxdensity as closely as possible corresponding to that of the maximumenergy product of the material. The maximum flux in a magnetic circuitoccurs in the magnet material approximately midway between the poles. Itis clear that the series magnet arrangement of the present inventionprovides a natural reduction in the cross sectional area adjacent thepole pieces (magnetic slabs 41) and a substantially larger crosssectional area midway between the poles (magnetic slabs 44) in theregion of maximum flux. A relatively uniform flux density thereforeexists at all sections of the magnetic circuit.

For a drum type magnetic separator of the type being discussed, adiameter of 30 to 36 inches frequently is used and the drum may be asmuch as 4 to 6 feet in length. The individual permanent magnets usedwould be func: tionally of substantially similar length, though thatlength can be achieved by using a plurality of magnets each of a smallerlength so that in the aggregate the 4 to 6 feet of magnets is attained.Of course, the pole pieces also extend the full length, and similarly,the full length required may be achieved by using a plurality of smallerpole pieces in contact with each other.

The materials used to produce the permanent magnets and pole pieces inaccordance with this invention can vary widely, as is apparent to thoseskilled in the art. Preferably, they are made from ceramic permanentmagnet high coercive force material such as the barium ferrites, orstrontium ferrites or other materials of like character, since thearrangement provides a large magnet area in relation to the magnetlength. However, the metallic permanent magnet materials such as Alnico,and the like also can be used. Similarly, the pole pieces can be made ofany soft magnetic material desired. Particularly, suitable are iron andcommon soft steel.

While the invention has been described primarily as employing orientedmagnetic material, and indeed this is the preferred material, it is alsocontemplated that isotropic high coercive force permanent magneticmaterial may be used in the magnetic assemblies of the invention. Inusing such material, the arrangement is such that directions of strongmagnetization of the permanent magnetic material essentially impart onepolarity thereof to one of the pole pieces of the magnet assembly, andthe other polarity to the other pole piece.

As has already been indicated, the pairs of magnets, 01'

pairs of stacks of magnets are magnetically coupled in series.Consequently, if the south pole of one magnet (or stack) is adjacent onepole piece, the north pole of the same magnet is adjacent the yoke,while for the companion magnet (or stack) of the same magnet assembly,the south pole is adjacent the yoke and the north pole is adjacent theother pole piece. The direction of magnetization'of adjacent stacks ofmagnets is shown by arrows 101 in FIG. 2. The two magnets or stacks ofmagnets of each magnet assembly are magnetically in series due to theiropposite magnetic orientation and to the presence of the yoke whichprovides a low reluctance path between the stacks.

In consequence of the just described arrangement of pole pieces andmagnets, internal flux leakage is minimal because it occurs only at theends of the magnet assemblies, along one side of each flank magnetassembly, and at bolt locations and mismatched magnet joints. Moreover,there is little flux leakage rearwardly of the magnet arrangement sincethe flux behind the magnets is concentrated in the yokes and isconducted by the yokes from one magnet (or stack) to the other. Hence,the flux is principally located in front of the magnets in an arebetween the pole pieces and thus substantially all of it, for practicalpurposes, is available to effect the desired separation.

Operation of a magnetic separator as just described follows that ofprior art devices. Thus, where an ore is being separated from a gangue,the ore that is to be re solved is c-omminuted to a relatively smallsize, for example, from fines ranging up to particles of A to /2 inch ormore. For wet operation, the finely divided ore, is dispersed in acarrier fluid, normally water, and fed to the separator through theinlet conduit. Wit-h the conveyor or drum surface rotating, the feedpasses into contact with it and the magnetic material is attracted toand held on the drum surface. The remaining materials pass to the firstoutlet conduit. The drum surface and its magnet assembly are of suchsize that the magnetic force is maintained on the particles on thesurface until they are carried beyond the area in which they would passoutwardly with the tailings. Once they are beyond that point, the end ofthe magnetic chuck or separator arrangement is reached and the particlesfall away from the drum surface under the influence of gravity and thefailure, due to remoteness, of sufficient magnetic force to retain them.The falling magnetic particles are directed to a separate outlet conduitprovided for them. The alternate polarities of the magnetic poles towhich the particles are subjected as the drum surface passes relative tothe magnetic drum arrangement, tends to vibrate these particles on thesurface, reorientating them and permitting entrapped nonmagneticmaterial to fall away.

In the resolution of materials with a separator as in this invention bywhat can be termed a dry process, the feed usually is arranged to passover the rotating surface under the influence of gravity. Normally fordry separations, more magnetic assemblies are used, often covering onthe order of 50 percent or more of the surface of the separating drum.In such practice, the feed may be introduced into contact with theseparating drum near its top and the feed inlet, commonly a simplefunnel arrangement to which a vibrator may be attached if necessary ordesirable, is supported from the frame above the drum. The tailings fallfree of the drum at its side to an outlet provided for them, while themagnetic particles are held to the surface beyond that point and areseparately removed.

Another arrangement, especially suited to dry separations or conveyingapplications, utilizes a drum having magnet assemblies along the entireinner surface of the sidewall of the drum. In such instance, the drumsidewall, which is driven, may provide all or part of the motive powerfor the conveyor.

Magnet assemblies as described are useful for many other industrialapplications. For example, they can be used in conjunction withconveyors by which cans may be held to a conveyor and moved as desired.Further, they are useful as spreaders of metal sheets arranged instacks.

From the foregoing discussion and description, it is apparent that thepresent invention is a significant advance in magnet assemblies. The Vshape of the element assembly contributes a mechanical advantage ofstrength and a structural advantage of mounting simplicity in additionto the magnetic benefits already noted. These advantages can be attainedeven with variations of the details of the invention. Thus, it will beevident that sizes, number of elements and the like can be changed.Furthermore, the face of the magnet assembly can be made flat or evenconcave, rather than convex as shown, for particular applications. Tipscan be added to the pole pieces to aid in directing the flux as desired.In all events, the materials of construction for different parts ofapparatus with which the magnet assembly is used should be chosen suchthat no materially adverse influence on the utilization of essentiallyall the available flux exists.

Having illustrated and described the invention in detail for purposes ofexemplification, it should be understood that the invention may bepracticed otherwise and is not to be limited by the details expressed.

I claim as my invention:

1. An assembly of magnets for use in separating procedures and conveyingapplications comprising a pair of elongated magnetic pole pieces ofopposite polarity defining between them the working face of said magnetassembly, an elongated yoke member of low reluctance material having apair of convergent inclined surfaces extending the length thereof, theconvergence of said surfaces forming an apex in the direction of saidworking face, each of said elongated pole pieces lying parallel to andspaced from one of said inclined surfaces, apair of bodies of orientedpermanent magnet material each substantially filling the space betweenone of said inclined surfaces and the pole piece associated therewithand extending to the working face, the oriented permanent magnet bodiesbeing arranged with their polarities magnetically coupled in seriesthrough the yoke member.

2. The magnetic assembly of claim 1 wherein the angle between theconvergent surfaces is about 90 and the oriented permanent magneticmaterial consists of a plu rality of stacks of ceramic permanent magnetsin slab form.

3. A highly efficient magnetic assembly having a pair of magnetic polesat a working face thereof, comprising an elongated yoke of V-shape incross section composed of permeable soft magnetic material and havingthe arms of the V extending rearwardly away from the working face of themagnetic assembly, a pair of pole members vof permeable soft magneticmaterial disposed at the 'working face, each in fixed spaced relationand parallel to an arm of said yoke, at least a pair of orientedpermanent magnet bodies substantially completely filling the spacebetween the yoke and the pole members and extending to the working faceof the assembly, the arrangement and directions of strong magnetizationof the permanent magnet material being such as to couple said magnetbodies in series through the yoke whereby the pole members are ofopposite polarity.

4. The magnetic assembly of claim 3 wherein the angle I between the armsof the yoke is about 90 and the oriented permanent magnetic materialconsists of a plurality of stocks of ceramic permanent magnets in slabform.

5. In a magnetic separator comprising a conveyor having a materialreceiving surface for conveying materials to be separated and apermanent magnet assembly having two poles at a working face thereof toattract magnetic material to be separated towards said surface, theconveyor being movable relative to the permanent magnet assembly, theimprovement comprising forming said assembly of a V-s'haped yoke ofpermeable soft magnetic material having two arms extending rearwardlyaway from the working face of the magnetic assembly, a pair of polemembers of permeable soft magnetic material disposed at the workingface, each in fixed spaced relation and parallel to an armof said yoke,at least a pair of oriented permanent magnet bodies substantiallycompletely filling the space between the yoke and the pole members andextending to the working face of the magnetic assembly, the arrangementand direction of strong magnetization of the oriented permanent magnetmaterial being such as to couple said mag-net bodies in series throughthe yoke whereby said pole members are of opposite polarity.

6. In a magnetic separator in accordance with claim 5, at least two suchassemblies of permanent magnets arranged so that adjacent assembliesshare a common pole, the oriented permanent magnetic material consistingof a plurality of stacks of ceramic permanent magnets in slab form andthe working faces of said assemblies being closely adjacent the surfaceof said conveyor.

7. In a magnetic separator, a drum having a cylindrical sidewallcomposed of a non-magnetic material and mounted for rotation on a shaft,a permanent magnet assembly having two poles at a working face thereofadapted to attract magnetic material to be separated towards theexternal surface'of the sidewall of the drum and mounted within thedrum, said working face being positioned in close proximity to theinternal surface of said sidewall, the magnetic assembly comprising aV-shaped yoke of permeable soft magnetic material having two armsextending away from the working face thereof, a pair of pole members ofpermeable soft magnetic material each spaced from and parallel to an armof said yoke, bodies of oriented permanent magnetic materialsubstantially completely filling the spaces between the arms of the yokeand the pole pieces and extending to the working face of the magneticassembly, the arrangement and direction of magnetization of the orientedpermanent magnet material being such as to couple said magnetic bodiesin series through said yoke.

8. In a magnetic separator in accordance with claim 7, at least two suchpermanent magnet assemblies adjacent one another and sharing a commonpole, the oriented permanentmagnet bodiesconsisting of ceramic permanentmagnet material.

9. An assembly of magnets for use in separating procedures and conveyingapplications comprising a generally V-shaped yoke having about a anglebetween the arms thereof, a pair of magnetic pole pieces external ofsaid V-shaped yoke, each pole piece lying parallel to and spaced fromone of said arms, at least a pair of permanent magnets between said yokeand said pole pieces, said magnets substantially filling the spacebetween said pole pieces, said magnets magnetized in directionsperpendicular to the arm of the yoke with which they are associated andarranged with their polarities magnetically coupled in series.

10. An assembly of magnets for use in separating procedures andconveying applications comprising a generally elongated yoke of lowreluctance material having a pair of convergent inclined surfacesextending the length thereof, the convergence of said surfaces formingan apex,ex-tending toward the working face of said assembly, a pair ofspaced magnetic pole pieces at said working face, each pole piece lyingparallel to and spaced from one of said inclined surfaces, at least apair of permanent magnets between said yoke and said pole pieces, saidmag- I nets being in contact with said yoke and said pole piecesReferences Cited by the Examiner UNITED STATES PATENTS 2,275,880 3/1942Arey 317201 2,535,719 12/1950 Blind 209-219 2,903,329 9/1959 Weber317201 X 3,146,191 8/ 1964 Greenwald 209-223 3,209,912 10/1965 Sloan209223 FRANK W. LUTTER, Primary Examiner,

HARRY B. THORNTON, Examiner.

R. HALPER, Assistant Examiner,

7. IN A MAGNETIC SEPARATOR, A DRUM HAVING A CYLINDRICAL SIDEWALLCOMPOSED OF A NON-MAGNETIC MATERIAL AND MOUNTED FOR ROTATION ON A SHAFT,A PERMANENT MAGNET ASSEMBLY HAVING TWO POLES AT A WORKING FACE THEREOFADAPTED TO ATTRACT MAGNETIC MATERIAL TO BE SEPARATED TOWARDS THEEXTERNAL SURFACE OF THE SIDEWALL OF THE DRUM AND MOUNTED WITHIN THEDRUM, SAID WORKING FACE BEING POSITIONED IN CLOSE PROXIMITY TO THEINTERNAL SURFACE OF SAID SIDEWALL, THE MAGNETIC ASSEMBLY COMPRISING AV-SHAPED YOKE OF PERMEABLE SOFT MAGNETIC MATERIAL HAVING TWO ARMSEXTENDING AWAY FROM THE WORKING FACE THEREOF, A PAIR OF POLE MEMBERS OFPERMEABLE SOFT MAGNETIC MATERIAL EACH SPACED FROM AND PARALLEL TO AN ARMOF SAID YOKE, BODIES OF ORIENTED PERMANENT MAGNETIC MATERIALSUBSTANTIALLY COMPLETELY FILLING THE SPACES BETWEEN THE ARMS OF THE YOKEAND THE POLE PIECES AND EXTENDING TO THE WORKING FACE OF THE MAGNETICASSEMBLY, THE ARRANGEMENT AND DIRECTION OF MAGNETIZATION OF THE ORIENTEDPERMANENT MAGNET MATERIAL BEING SUCH AS TO COUPLE SAID MAGNETIC BODIESIN SERIES THROUGH SAID YOKE.